Improvements in magnesium diboride superconductors and methods of synthesis

ABSTRACT

Improved magnesium diboride superconducting materials and methods of synthesis are disclosed. Embodiments of the superconducting material comprise at least two starting materials capable of forming MgB 2  and at least one dopant compound comprising silicon, carbon, hydrogen and oxygen. The starting materials and the at least one dopant compound are heated and mixed at an atomic level to produce a silicon-doped MgB 2  superconducting material. Examples of the dopant compound include silicone oil, Triacetoxy(methyl)silane ( 2 ), 1,7-Dichloro-octamethyltetrasiloxane ( 2 ) and Tetramethyl orthosilicate ( 6 ).

FIELD OF THE INVENTION

The present invention relates to superconducting materials and methodsof synthesis thereof. In particular, the present invention relates todoped superconducting materials comprising magnesium diboride (MgB₂) andmethods of synthesis thereof.

BACKGROUND TO THE INVENTION

Superconductors have two important characteristics distinguishing themfrom other materials such as semiconductors, metallic conductors etc.One is losing their resistance and the other is repelling magnets ormagnetic fields or levetating above magnets when they are in asuperconducting state. Therefore, superconductors have significantapplications as superconducting cables being able carrying very largeelectric currents without energy loss, and as superconducting magnetsproducing much higher magnetic fields than conventional electromagnets.

For a material to exhibit superconducting behaviour, the material mustbe cooled below its critical temperature (T_(c)), the current passingthrough the cross-section of the material must be below the criticalcurrent density (J_(c)) and the magnetic field to which the material isexposed must be below the critical magnetic field (H_(c)). Magnesiumdiboride (MgB₂) is a superconductor with a much higher superconductingtransition temperature (T_(c)) of 40 K and lower cost than conventionallow temperature superconductors (T_(c)<25 K) and is of great potentialfor large-scale and microelectronic applications at temperatures farabove that of liquid helium (T=4.2 K).

For practical applications that require carrying large supercurrents inthe presence of magnetic fields, improvements in the critical currentdensity (J_(c)), the upper critical field (H_(c2)), and theirreversibility field (H_(irr)) have been the key topics of research onMgB₂ superconductors. An effective way to improve the flux pinning is tointroduce flux pinning centres into MgB₂. It has been found thatchemical doping with non-magnetic materials appears to be the mostsuitable approach to increase the ability of MgB₂ to carry largecurrents for practical applications. A number of additives have beenexamined for J_(c), H_(c2), and H_(irr) improvements. It has alreadybeen shown that a J_(c) enhancement by more than one order of magnitudein high magnetic fields can be easily achieved with only a slightreduction in T_(c) through doping MgB₂ with nanoparticles, such as SiC,Si, and C. It has also been shown that SiC doping significantly enhancesthe H_(c2) and H_(irr) in polycrystalline bulks, as well as in wires andtapes.

For C doping, high sintering temperatures are required to allow the C toreadily substitute for B. The partial replacement of B by C is believedto be responsible for the enhancement of H_(c2) and flux pinning in MgB₂according to the two band scattering model. However, a low sinteringtemperature is much more desirable for practical applications. Itsadvantages include reducing the reaction between metal sheath materialsand MgB₂, lower fabrication costs and making finer MgB₂ grains. Nano-SiCor Si doping can effectively enhance the flux pinning in MgB₂ even whenthe samples are processed at temperatures as low as around 600° C.

The improvement of flux pinning enhancement is controlled by the sizesof the particles doped into the MgB₂. However, the requirement for finernanoparticles brings some dilemmas, such as higher cost and sometechnical problems in fabricating the much finer nanoparticles. Becausethe nanoparticles are in solid state form, another problem isagglomeration of nanoparticles, which limits the homogeneity of mixingwith MgB₂. This homogeneity of mixing is very crucial in determining theflux pinning ability for MgB₂ made by the in-situ reaction method.Recently, it has been reported that aromatic hydrocarbon addition toMgB₂ can enhance the flux pinning in MgB₂ at low sintering temperatures.However, the enhancement is not greater than in nano-SiC doped samples,and this organic solvent is very volatile at ambient pressure.

In addition, solid state malic acid addition into MgB₂ has also beenreported to enhance the flux pinning in MgB₂. However, the sinteringtemperature used was as high as 900° C. for 30 min.

Hence, there are one or more deficiencies in the known low temperaturesuperconductor materials incorporating MgB₂. Because of the commercialappeal of superconductors comprising MgB₂ there is a need to address orat least ameliorate one or more of these deficiencies or provide asuitable commercial alternative thereto.

In this specification, the terms “comprises”, “comprising” or similarterms are intended to mean a non-exclusive inclusion, such that amethod, system or apparatus that comprises a list of elements does notinclude those elements solely, but may well include other elements notlisted.

SUMMARY OF THE INVENTION

In one form, although it need not be the only or indeed the broadestform, the invention resides in a superconducting material comprising:

at least two starting materials capable of forming MgB₂; and at leastone dopant compound comprising silicon, carbon, hydrogen and oxygen;

wherein the starting materials and the at least one dopant compound areheated and mixed at an atomic level to produce a silicon-doped MgB₂superconducting material.

Suitably, the MgB₂ superconducting material further comprises one ormore of the following in the MgB₂ lattice: carbon doping; oxygen doping.

Preferably, the at least one dopant compound is a liquid, but may alsobe a solid.

Suitably, the at least one dopant compound is a siloxane and is in theform of silicone oil (—SiC₂H₆O—)_(n).

Suitably, the at least one dopant compound includes, but is not limitedto, one or more of the following: Triacetoxy(methyl)silane (2);(CH₃CO₂)₃SiCH₃; 1,7-Dichloro-octamethyltetrasiloxane (2) C₈H₂₄Cl₂O₃Si₄;Tetramethyl orthosilicate (6) Si(OCH₃)₄.

In another form, although again not necessarily the broadest form, theinvention resides in a superconducting material comprising:

at least two starting materials capable of forming MgB₂; and

at least one dopant compound comprising silicon, carbon and hydrogen;

wherein the starting materials and the at least one dopant compound aremixed at an atomic level and heated to produce oxygen or anoxygen-containing compound at an intermediate stage and a silicon-dopedMgB₂ superconducting material.

Suitably, the at least one organic dopant compound includes, but is notlimited to, one or more of the following: Tetrakis(trimethylsilyl)silane(1), [(CH₃)3Si]4Si, which sublimes to produce CO, CO₂ and SiO₂ in air;Hexamethyldisilane (1), (Si(CH₃)3)2; Tetraethylsilane (2) Si(C₂H₅)4.

In another form, although again not necessarily the broadest form, theinvention resides in a method of synthesizing a superconducting materialincluding:

a) mixing at least two starting materials capable of forming MgB₂ withat least one dopant compound comprising silicon, carbon, hydrogen andoxygen; and

b) heating the mixed materials such that the at least two startingmaterials and the at least one dopant compound react at an atomic levelto produce a silicon-doped MgB₂ superconducting material.

Suitably, the at least one dopant compound represents ≦30 wt % of MgB₂and in some embodiments represents 3, 10, 15, 20, or 30 wt % of MgB₂.

Further forms and features of the present invention will become apparentfrom the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

By way of example only, preferred embodiments of the invention will bedescribed more fully hereinafter with reference to the, accompanyingdrawings, wherein:

FIG. 1 is a general flow diagram showing a method of synthesizing asuperconducting material in accordance with embodiments of the presentinvention;

FIG. 2 is an X-ray diffraction pattern for an embodiment of thesuperconducting material with the variation of the a and c latticeparameters with doping level shown in the inset;

FIG. 3 shows resistance versus temperature (R-T) curves for embodimentsof the superconducting material with further detail shown in the insets;

FIG. 4 is a graph showing the magnetic field dependence of the criticalcurrent density at different temperatures for embodiments of thesuperconducting material;

FIG. 5 shows graphs illustrating the variation in upper critical fieldand irreversibility field as a function of normalized temperature fordifferent levels of doping according to embodiments of thesuperconducting material;

FIG. 6 is a graph illustrating the field dependence of the volumepinning force at 20K for embodiments of the superconducting material;and

FIG. 7 is a graph showing the full width at half maximum (FWHM) ofvarious diffraction peaks as a function of the doping level inaccordance with embodiments of the superconducting material.

DETAILED DESCRIPTION OF THE INVENTION

To solve the aforementioned problem of nanoparticle agglomeration,embodiments of the present invention use precursors, preferably inliquid form, that contain at least Si and C that are able to introduceboth Si and C into MgB₂ at an atomic scale, even when the sintering timeis short and at low temperatures.

According to one embodiment, the starting materials capable of formingthe superconducting material MgB₂ are amorphous boron powder with apurity of 99.9% and Mg powder with a purity of 99%. These are mixed witha dopant compound comprising silicon, carbon, hydrogen and oxygen in theform of commercial, high temperature silicone oil from Sigma Aldrich.Commercial silicone oil, (—SiC₂H₆O—)_(n), is a colourless, odourless,chemically inert lubricant, with excellent thermal stability.

With reference to the method 100 of synthesizing embodiments of thesuperconducting material shown in FIG. 1, at 110 the B and Mg powders atchemical stoichiometry are thoroughly mixed with diluted silicone oil inacetone. A range of samples with different doping levels were produced.The amounts of silicone oil added into the MgB₂ samples were 3, 10, 15,20, and 30 wt %. At 120, the samples were shaped into pellets 13 mm indiameter and 2 mm in thickness under uniaxial pressure. At 130, thesepellets were then sealed in an iron tube and at 140 sintered in a tubefurnace at 750-780° C. for 10 min only. It has been found that shortsintering is as good as long sintering in terms of flux pinning forMgB₂. A high purity argon gas flow was maintained throughout the in situsintering process to avoid oxidation. An undoped MgB₂ sample was alsoprepared under the same in situ processing conditions as a referencesample.

It will be appreciated that the aforementioned method is forexperimentation purposes. For commercial applications, known powder intube wire drawing techniques can be employed to produce superconductingwires in accordance with embodiments of the superconducting materialdescribed herein. Other known techniques can be employed to produce thesuperconducting material in other shapes, such as tapes and in bulk.

From x-ray diffraction (XRD) experiments, it was observed that all thesamples crystallized in the MgB₂ structure as the major phase. Slightamounts of MgO and Mg₂Si are also present in the silicone oil dopedsamples. The amount of Mg₂Si is increased by increasing the silicone oilcontent. However, the tiny amount of MgO phase remains the same for theundoped sample and all the doped samples as determined by XRD.

The decomposition of pure commercial silicone oil possibly follows thefollowing reaction at 800° C.:

(—SiC₂H₆O—)_(n)→SiO+2C+3H₂→SiC+CO.

The aforementioned decomposition of silicone oil took place below 800°C. because all the samples were sintered at 780° C. Si and C released asa result of the decomposition of the silicone oil may not form SiC, asno detectable SiC phase was observed from the XRD patterns. It isbelieved that the chemically active Mg reacted with Si and that thiscaused the decomposition of silicone oil at relatively low temperatures.The remaining C would then embed itself into the MgB₂ grains togetherwith Mg₂Si and also substitute into B sites in the MgB₂ crystal lattice,as has been observed in nano-SiC, Si, and C doped MgB₂.

The calculated XRD patterns using Rietveld refinement fit very well withthe observed patterns. The refined and observed XRD patterns for the 10wt % silicone oil added sample are shown in FIG. 2 with the variation ofthe a and c lattice parameters with doping level shown in the inset.(The arrows in the inset point to the respective lattice parameter.) Thelattice parameters obtained by the refinement revealed that the alattice parameter is reduced from 3.085 to 3.065 Å for the pure and 15wt % silicone oil doped samples, respectively, while the c latticeparameter is only slightly increased, as illustrated in the inset.

The significant reduction in the a lattice parameter indicates thatcarbon has been doped into the B sites in the crystal lattice and causedthe reduction in T_(c). Both C doping and the inclusion of Mg₂Si canenhance the electron scattering, as proved by the decreased residualresistivity ratio (RRR) values, and, in turn, enhance the flux pinning.

FIG. 3 shows the resistance versus temperature curves (R-T) for threesamples at zero external magnetic field over a temperature range of30-300 K. It can be seen that the scattering increases with increasingsilicone oil content. The resistivity at 40 K increases from 24 μΩ cmfor the pure MgB₂ to 64 μΩ cm for the 10 wt % silicone oil doped MgB₂.The T_(c) values and residual resistivity ratios, R(300K)/R(Tc), wereobtained to be 38.2K, 37K, and 36.2 K and 2.72, 2.0, and 1.67, for the 0wt %, 3 wt %, and 10 wt % silicone oil samples, respectively.

The magnetic field dependence of J_(c) at 30, 20, and 5 K is shown inFIG. 4. It should be noted that the J_(c) values in high fields aresignificantly enhanced for all the doped samples. The J_(c) of theun-doped sample dropped to 100 A/cm² at 7 T and 5 K. However, the J_(c)values at the same field are increased by more than one or two orders ofmagnitude for the 3, 10, and 15 wt % silicone oil added samples. At 8 Tand 5 K, the Jc values of the 10 and 15 wt % doped samples are over(1-2)×10⁴ A/cm², more than one order of magnitude higher than for the 3wt % doped sample. It should also be noted that there was no degradationin self-field J_(c) values for the 10 and 15 wt % silicone oil dopedsamples.

The H_(c2) and H_(irr) were also enhanced, as proved by the datadetermined from the R-T curves, which are shown in the inset of FIG. 3.The inset shows the resistance versus temperature (R-T) measured atdifferent applied magnetic fields up to 8.7 T for the 10 wt % dopedsample.

The H_(c2) values versus normalized temperature T/T_(c) obtained fromthe 90% or 10% values of their corresponding resistive transitions areshown in FIG. 5. The H_(c2) values of the undoped sample are alsoincluded for comparison. Significantly enhanced H_(irr) and H_(c2) forthe silicone oil doped sample are clearly observed. It can be seen thatthe H_(c2) curves of all the samples show a positive curvature nearT_(c) as a result of the two band superconductivity in MgB₂. Also, allthe doped samples have larger dH_(c2)/d(T/Tc) values compared to theundoped sample. The evolution of the enhancement of flux pinning isshown clearly in the variation of the ratior(H_(irr))=H_(irr)(doped)/H_(irr)(undoped) orr(H_(c2))=H_(c2)(doped)/H_(c2)(undoped) with T/T_(c). (The arrows inFIG. 5 point to the respective axes for these variations.) Both ratiosare about 1.25 and 1.5 for the 3 wt % and the 10 wt % silicone oil dopedMgB₂, respectively. The above results reveal that MgB₂ with silicone oiladded exhibits higher H_(irr) values compared to the undoped samplesthat were processed under the same fabrication conditions.

The field dependence of the normalized volume pinning force F_(p)=J×B at20 K for all the samples is shown in FIG. 6. It can be seen that thepinning force for the silicone oil added samples is significantly higherthan for the undoped sample at B>1.5 T. The XRD diffraction peaks areobserved to broaden with an increasing amount of silicone oil. FIG. 7shows the full width at half maximum (FWHM) for the (100), (002), and(110) peaks for all the samples. It can be seen that the values of theFWHM of the (100) peak increase monotonically for all samples with anamount of Si oil up to 15 wt %. The FWHM values also increase for the(002) and (110) peaks for the 3 and 10 wt % silicone oil samples. Thepeak broadening in these samples likely arises from non-uniform strainthat is mainly caused by C doping on B sites. The grain sizes, whichcould also affect the peak width, have been observed to be very similarunder scanning electron microscopy. However, a further study on thegrain sizes and crystal defects using high resolution transmissionelectron microscopy is needed. The presence of Mg₂Si impurity phase isalso responsible for the peak broadening, as the Mg₂Si is believed toact as a grain refiner in MgB₂. Therefore, the enhanced flux pinning,H_(c2), H_(irr), and J_(c)(H) observed in our silicone oil added MgB₂are due to the C-doping effect and inclusions of Mg₂Si. It is believedthat the large distortion of the crystal lattice caused by both carbonsubstitution for B and inclusion of Mg₂Si leads to enhanced electronscattering and enhancement of H_(c2).

The data on SiC nanopowder added MgB₂ prepared using a hot pressingmethod presented in our previous work are better than what we haveachieved in this work using Si oil. However, it is easier and cheaper toenhance the flux pinning with Si oil compared to using SiC nanopowders.Further improvement of the flux pinning performance of MgB₂ using Si oilis highly possible by optimizing the processing conditions.

In summary, it has been found that a significant flux pinningenhancement in MgB₂ can be easily achieved using a liquid additive,silicone oil. The results showed that Si and C released from thedecomposition of the silicone oil formed Mg₂Si and substituted into theB sites, respectively. Increasing the amount of Si oil up to 15 wt %leads to the reduction of the lattice parameters, as well as T_(c) andR(300 K)/R(T_(c)) values, resulting in a significant enhancement ofJ_(c)(H), H_(irr), and H_(c2).

In alternative embodiments, the starting materials capable of formingMgB₂ can include one or more powders of the following MgB₂, MgH₂, MgB₄.It is also envisaged that flux pinning enhancement and enhancement ofJ_(c)(H), H_(irr), and H_(c2) can also be achieved with lower puritystarting materials.

Although the dopant compound in the aforementioned embodiments is aliquid, in alternative embodiments, the dopant can be a solid or apowder, which is dissolved in a solvent, such as acetone, toluene,hexane, benzene or other solvent.

In alternative embodiments, a sintering temperature of about 600-1000°C. and a sintering time of about a few minutes up to about 24 hours canbe employed.

In other embodiments, other dopant compounds comprising silicon, carbon,hydrogen and oxygen can be employed, which can be in the form of, forexample, other siloxanes, such as, but not limited to,1,7-Dichloro-octamethyltetrasiloxane (2) C₈H₂₄Cl₂O₃Si₄ and can bepolymerized siloxanes.

In further embodiments, the dopant compound can be a silane, such as,but not limited to, Triacetoxy(methyl)silane (2); (CH₃CO₂)₃SiCH₃ or asilicate, such as, but not limited to, Tetramethyl orthosilicate (6)Si(OCH₃)₄.

In other embodiments, the dopant compound comprises silicon, carbon andhydrogen. In accordance with embodiments of the present invention, whenone or more such dopant compounds are mixed with the starting materialscapable of forming MgB₂ and heated to mix the constituents at an atomiclevel, as described in the aforementioned method, oxygen, or one or moreoxygen-containing compounds, are produced at an intermediate stage, toultimately produce a silicon-doped MgB₂ superconducting material. Forexample, the organic dopant compound can include, but is not limited to,one or more of the following: Tetrakis(trimethylsilyl)silane (1),[(CH₃)3Si]4Si, which sublimes to produce CO, CO₂ and SiO₂ in air,Hexamethyldisilane (1), (Si(CH₃)3)2 or Tetraethylsilane (2) Si(C₂H₅)4.Silicon-doped MgB₂ superconducting materials produced using one or moreof the alternative dopants recited above are also likely to exhibitC-doping effects and inclusions of Mg₂Si to provide flux pinningenhancement and enhancement of J_(c)(H), H_(irr), and H_(c2).

In yet further embodiments, instead of one or more of the aforementionedorganic dopant compounds being employed to produce a doped MgB₂superconducting material, the dopant compound can include, but is notlimited to, one or more of the following: SiCl₄, Sil₄, CCl₄, Cl₄, fineSi, SiO₂, SiC.

Hence, the superconducting materials and methods of synthesis of thepresent invention address the agglomeration problem of the prior artbecause silicone oil and the other dopants referred to herein areliquids or are diluted in a solvent this enabling the dopant to mix withthe starting materials and thus with MgB₂ very homogeneously. Only asmall reduction in T_(c) compared to some of the prior art dopants isobserved, whilst enhanced flux pinning and J_(c)(H), H_(irr), and H_(c2)values are observed. The dopants described herein are cheaper thannano-SiC and CNTs and easier to work with and can produce superior MgB₂superconducting materials at lower temperatures.

Throughout the specification the aim has been to describe the inventionwithout limiting the invention to any one embodiment or specificcollection of features. Persons skilled in the relevant art may realizevariations from the specific embodiments that will nonetheless fallwithin the scope of the invention.

1. A superconducting material comprising: at least two startingmaterials capable of forming MgB₂; and at least one dopant compoundcomprising silicon, carbon, hydrogen and oxygen; wherein the startingmaterials and the at least one dopant compound are heated and mixed atan atomic level to produce a silicon-doped MgB₂ superconductingmaterial.
 2. The superconducting material of claim 1, further comprisingone or more of the following in the MgB₂ lattice: carbon doping; oxygendoping.
 3. The superconducting material of claim 1, wherein the at leastone dopant compound is a liquid.
 4. The superconducting material ofclaim 1, wherein the at least one dopant compound is a siloxane.
 5. Thesuperconducting material of claim 4, wherein the siloxane ispolymerized.
 6. The superconducting material of claim 1, wherein the atleast one dopant compound is silicone oil (—SiC₂H₆O—)_(n).
 7. Thesuperconducting material of claim 1, wherein the at least one dopantcompound includes one or more of the following: Triacetoxy(methyl)silane(2); (CH₃CO₂)₃SiCH₃; 1,7-Dichloro-octamethyltetrasiloxane (2)C₈H₂₄Cl₂O₃Si₄; Tetramethyl orthosilicate (6) Si(OCH₃)₄.
 8. Thesuperconducting material of claim 1, wherein the at least one dopantcompound represents ≦30 wt % of MgB₂.
 9. The superconducting material ofclaim 1, wherein the at least one dopant compound represents 3, 10, 15,20, or 30 wt % of MgB₂.
 10. A superconducting material comprising: atleast two starting materials capable of forming MgB₂; and at least onedopant compound comprising silicon, carbon and hydrogen; wherein thestarting materials and the at least one dopant compound are mixed at anatomic level and heated to produce oxygen or an oxygen-containingcompound at an intermediate stage and a silicon-doped MgB₂superconducting material.
 11. The superconducting material of claim 10,comprising one or more of the following in the MgB₂ lattice: carbondoping; oxygen doping.
 12. The superconducting material of claim 10,wherein the at least one dopant compound includes, one or more of thefollowing: Tetrakis(trimethylsilyl)silane (1), [(CH₃)3Si]4Si;Hexamethyldisilane (1), (Si(CH₃)3)2; Tetraethylsilane (2) Si(C₂H₅)4. 13.The superconducting material of claim 10, wherein the at least onedopant compound represents ≦30 wt % of MgB₂.
 14. The superconductingmaterial of claim 10, wherein the at least one dopant compoundrepresents 3, 10, 15, 20, or 30 wt % of MgB₂.
 15. A method ofsynthesizing a superconducting material including: a) mixing at leasttwo starting materials capable of forming MgB₂ with at least one dopantcompound comprising silicon, carbon, hydrogen and oxygen; and b) heatingthe mixed materials such that the at least two starting materials andthe at least one dopant compound react at an atomic level to produce asilicon-doped MgB₂ superconducting material.
 16. The method of claim 15,further including heating the mixed materials for about several minutesup to 24 hours.
 17. The method of claim 15, further including heatingthe mixed materials at 600-1000° C.
 18. The method of claim 15, furtherincluding dissolving the at least one dopant compound in acetone,toluene, hexane, benzene or other solvent.
 19. The method of claim 15,wherein the at least one dopant compound includes one or more of thefollowing: a siloxane; Triacetoxy(methyl)silane (2); (CH₃CO₂)₃SiCH₃;1,7-Dichloro-octamethyltetrasiloxane (2) C₈H₂₄Cl₂O₃Si₄; Tetramethylorthosilicate (6) Si(OCH₃)₄.
 20. A method of synthesizing asuperconducting material including: a) mixing at least two startingmaterials capable of forming MgB₂ with at least one dopant compoundcomprising silicon, carbon and hydrogen; and b) heating the mixedmaterials such that the at least two starting materials and the at leastone dopant compound react at an atomic level to produce oxygen or anoxygen-containing compound at an intermediate stage and a silicon-dopedMgB₂ superconducting material.
 21. The method of claim 20, wherein theat least one dopant compound includes one or more of the following:Tetrakis(trimethylsilyl)silane (1), [(CH₃)3Si]4Si; Hexamethyldisilane(1), (Si(CH₃)3)2; Tetraethylsilane (2) Si(C₂H₅)4.
 22. The method ofclaim 20, further including heating the mixed materials for aboutseveral minutes up to 24 hours.
 23. The method of claim 20, furtherincluding heating the mixed materials at 600-1000° C.
 24. The method ofclaim 20, further including dissolving the at least one dopant compoundin acetone, toluene, hexane, benzene or other solvent.